The invention concerns a one-piece expandable flat bearing structure. That bearing structure is formed by at least partially elastically deformable struts which are separated from each other by openings in the bearing structure. The bearing structure can assume at least one compressed condition and at least one expanded condition and has at least one expansion direction into which the bearing structure can expand from the compressed condition to the expanded condition.
The invention concerns in particular endoluminal prostheses and in particular stents with such a bearing structure.
The term flat bearing structure is here also used to denote such a bearing structure which forms, for example, the peripheral wall of a tubular article which is open at its two ends. Such a tubular article which is open at both ends is represented by per se known stents.
Stents are used in many cases as implantable vessel supports. Known stents are urethra stents as well as coronary stents and peripheral stents. Coronary stents serve to deal with vessel constrictions in the coronary blood vessels, so-called stenoses, by means of a radial supporting action which emanates from the stent. The same is implemented by peripheral stents for peripheral blood vessels.
Many stents of that kind are produced from a one-piece tube as the starting material by cutting, for example, by means of a laser. The cuts into the tube serving as the starting material provide a bearing structure of struts and openings which are disposed therein, which structure can be radially expanded in the case of stents. In particular, the expanded bearing structure is grid-like and has struts which enclose radial openings of greater or lesser size in the stent. Balloon-expandable stents and self-expandable stents are known. Self-expandable stents comprise for example a memory metal such as nitinol which, upon reaching a jump temperature, jumps from a first shape corresponding to the compressed condition of the stent, to a second shape corresponding to the expanded condition of the stent.
The balloon-expandable stents, which are of particular interest in the present context, do not automatically change from the compressed to the expanded-condition but rather are expanded by means of a balloon which is pumped up with a fluid in the interior of the stents. A balloon of that kind is generally arranged at the end of a balloon catheter which serves at the same time, for introducing the stent as far as the location to be treated at the vessel constriction. For that purpose, the stent is crimped onto the balloon. The stent is expanded by means of the balloon at the treatment location.
The demands made on stents of that kind are many and various. In order on the one hand to be well introduced into a blood vessel and on the other hand to sufficiently expand a stenosis, the bearing structure of the stent must be capable of being expanded from a compressed diameter, which is as small as possible, to a sufficiently large expanded diameter. The expanded bearing structure must also enjoy a sufficient radially acting bearing force, also referred to as radial strength, in order to reliably hold the vessel open. Further desirable properties are suitable surface coverage by the expanded stent, as well as appropriate behavior upon flexing about the longitudinal axis of the stent, as is frequently required in relation to blood vessels in the human body. A further demand on bearing structures for stents is that a balloon-expandable stent is to spring back again as little as possible after expansion by the balloon. More specifically, the consequence of such spring-back effect on the part of the bearing structure after expansion by a certain amount is that the bearing structure has to be expanded by that amount beyond the desired final dimension so that ultimately the bearing structure assumes the desired final stent dimension. That spring-back effect on the part of the bearing structure immediately after expansion when the balloon serving for expansion is deflated again, is referred to as recoil.
The co-operation of material property and stent design lead to the structural properties of the stent. To assess a stent design and for comparison with other stents, reference is made to characteristic values which are set forth in the following section. In order to obviate misunderstandings, it should be noted at this juncture that this involves pure engineering characteristic values for describing the stent properties, which are not to be confused with the situation in the vessel.
Recoil:
Recoil describes the elastic spring-back effect during or after evacuation of the balloon catheter. It is defined as the relative difference between the diameter at maximum pressure and the diameter after balloon evacuation.
Radial strength:
Radial strength specifies the maximum external pressure that the stent withstands. The characteristic value is the collapse pressure which is defined as the pressure at which the stent has collapsed.
Shortening:
The stent can shorten during stent dilation. The parameter describing that phenomenon is called shortening and is defined as the relative difference between the length prior to dilation and the final length.
Flexural stiffness:
The coronary vessels generally do not extend in a straight line but are twisted so that the stent should be as flexurally soft as possible for easy passage to the implantation location. In addition, flexurally soft stent designs permit stent implantation in curved and branched coronary vessels [4]. The flexural stiffness of stents is ascertained as structural stiffness E1 with the unit [Nmm] from a flexural test [5].
Further parameters:
The expansion behavior, crimpability and compliance are further considered.
Expansion behavior is determined by the balloon pressure at which the stent opens, free deployment of the struts takes place and so forth. Crimpability influences handling of the stent from the point of view of the cardiologist or machine crimping in the case of the complete systems.
It is very difficult to arrive at a relationship between the mechanical characteristic values of a stent and its biocompatibility and specifically hemocompatibility. Biocompatibility is composed of surface compatibility and structural compatibility. In the case of a stent, surface compatibility involves blood contact on the one hand and tissue contact with the vessel wall on the other hand. Structural compatibility extends from the mechanical supporting effect by way of flexural stiffness and the strut shape to the fluidic influences on the blood flow. The stent should not destroy the vessel, it should not result either in mechanical or toxic irritations and it should be athrombogenic in terms of blood contact.
The person skilled in the art is aware of a large number of bearing structures for stents, which all afford various advantages and conversely frequently also entail certain disadvantages. The known bearing structures, for example, can frequently be embodied only insufficiently or not at all with the materials which have a low modulus of elasticity. Most of the known bearing structures presuppose materials which can be well plastically deformed. That is important, in particular, in regard to the above-indicated requirement for keeping recoil as low as possible.